In MR imaging, sequences consisting of RF signals (also referred to as B1) and switched magnetic field gradients are applied to an object (a patient) placed in a homogeneous magnetic field within an examination volume of an MR device. In this way, MR signals are generated, which are scanned by means of RF receiving antennas in order to obtain information from the object and to reconstruct images thereof. Since its initial development, the number of clinically relevant fields of application of MR imaging has grown enormously. MR imaging can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body. The imaging sequence, which is applied during an MR scan, plays a significant role in the determination of the characteristics of the reconstructed image, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of an MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.
In interventional and intraoperative MR imaging high-performance computing and novel therapeutic devices are combined. These techniques permit the execution of a wide range of interactive MR guided interventions and surgical procedures. A basic issue of interventional MR imaging is the visualization and localization of instruments or surgical devices. This can be done either using active techniques, e.g. by means of RF micro coils attached to the tip of an instrument, or passive localization techniques that rely on local magnetic susceptibility induced image artifacts.
The active localization approach allows the immediate determination of the instrument coordinates and therefore allows robust tracking of instruments. It further enables functionalities like, e.g., image slice tracking. A drawback of active localization is that it implies a safety issue due to the presence of electrically conductive cables which may act as RF antennas and which may lead to dangerous tissue heating.
WO 2005/103748 A1 discloses a way to suppress the hazards associated with the induction of currents in the electrically conductive cables that are used as transmission lines for connecting auxiliary means, such as interventional instruments or RF surface coils, to the MR system. According to the conventional approach, inductors are introduced into the connecting cable. These inductors are coupled such that they form a transformer. Additionally, a tuning and matching network is integrated into the cable resembling a tuned blocking filter. This arrangement suppresses induced currents that would lead to dangerous heating of the cable.
A drawback associated with the known transformer-based transmission lines integrated, for example, into catheters or guidewires is that it involves a considerable hardware effort to build these devices.
An alternative approach is proposed by Celik et al. (“A Novel Catheter Tracking Method Using Reversed Polarization”, in Proc. Intl. Soc. Mag. Reson. Med., vol. 14, 2006, page 264). Reverse circular polarisation is used to obtain an MR image of an RF coil which is attached as a resonant marker to an interventional instrument. Standard quadrature birdcage coils which are used as RF antennas in conventional MR systems are designed to receive only forward circularly polarised RF fields, because the protons in the examined body have a forward polarisation as well. This is due to the positive gyromagnetic ratio of the hydrogen nuclei. Therefore, a standard birdcage coil modified to receive only reverse circularly polarised RF signals would pick up no MR signal from the body at all. However, the RF coil attached to the interventional instrument picks up the MR signals from the examined body and radiates a linearly polarised RF field. A linearly polarised radiation can be considered as a superposition of a forward and a reverse circularly polarised RF field. Therefore, the modified quadrature birdcage coil, which is designed to receive only reverse polarised RF radiation, picks up the signal which is radiated from the RF coil attached to the interventional instrument, but no signal from the surrounding body tissue is obtained. In this way, a background-free image showing only the position of the interventional instrument is generated.
A drawback of the afore-described technique is that a specially designed receive-only quadrature body coil has to be used to pick up the reverse circularly polarised RF signals generated by the RF coil attached as a resonant marker to the interventional instrument. The RF coil is excited indirectly via the nuclear magnetization of the body tissue during the imaging and localisation procedure. Hence, the SNR (signal to noise ratio) is comparatively low, which is a further disadvantage of the known method.
A further option to provide a signal or power transmission path between an MR apparatus and an interventional device (or any other auxiliary equipment) without interfering with the MR imaging procedure is the use of off-resonant RF. However, off-resonant RF also has several disadvantages. Off-resonant RF can not be easily converted to on-resonant RF. Furthermore, additional RF transmission and reception means are required if the off-resonant RF is outside the bandwidth of the conventional RF equipment of the MR apparatus. On the other hand, if the off-resonant RF is within the bandwidth of the customary RF chain of the MR apparatus, unwanted interference with the nuclear spin system may occur.